As is known, coordinate measurement machines comprise members movable along coordinate axes for the purposes of moving a measuring sensor in a measurement volume. Typically, the movable members are constituted by a first carriage movable along a first axis, a second carriage carried by the first carriage and movable with respect thereto along a second axis orthogonal to the first one, and a spindle carried by the second carriage and movable with respect thereto along a third axis orthogonal to the first two.
In particular, the present method relates to the compensation of measurement errors due to the thermal deformation of the machine structural components that form the guideways for the movable members. These components are characterized by an elongated prismatic shape with a longitudinal dimension that is significantly larger than the others.
The components may constitute the movable part or the fixed part of the relevant slide axis.
Some non-limitative examples of these components are constituted, for example, by:                the vertical axis or spindle of a gantry or cantilever machine;        the horizontal axis or arm of a horizontal arm machine;        the vertical axis or upright of a horizontal arm machine.        
Typically, as shown in FIG. 1, the components 1 that form the slide axes in coordinate measurement machines, for example of the previously indicated type, are constituted by a structural element 2 and by one or more guide elements 3 (for example rails for recirculating-ball pads or rollers or guides for pneumostatic pads) mounted thereon. The structural element 2 and the guide element(s) 3 have an elongated prismatic shape and substantially the same length. The structural element 2 generally has a larger cross-sectional size, while the guide elements 3 are slimmer and more flexible.
The joint between the structural element 2 and the guide element(s) 3 is generally provided by threaded connections 4 sized in such a way that, for practical purposes, the component 1 may be considered as a single piece from the structural standpoint.
The assembled component 1 has a constant section along the longitudinal direction for substantially its entire length.
Depending on the position of the guide elements 3 on the section, the section may be asymmetric and the barycentre of the sections of the guide elements 3 might not coincide with the neutral axis of the section of the structural element 2.
Examples of asymmetric sections are shown in FIGS. 2 and 3: in FIG. 2, the structural element 2 is constituted by a hollow, quadrangular section beam and the guide elements are applied on a face of the structural element 2, along parallel edges of the latter; in FIG. 3, the guide elements 3 are arranged on two opposite faces, along edges adjacent to a third face.
In general, the structural element 2 and the guide element(s) 3 are made of different materials or, in the case where they are made of the same material (steel for example), the properties of the material may still be different due to different manufacturing technologies and the required functional characteristics.
In particular, the materials may differ with regard to the linear thermal expansion coefficient (CTE).
In use, the component is subjected to temperature changes within the range of permissible conditions for using the machine (for example, 15-35° C.). The temperature changes are intended as temporal variations; spatial temperature gradients along the component are not considered here and their effects might possibly overlap those of the temporal variations.
A temperature change causes a differential length change length between the structural element 2 and the guide elements 3, due to the different linear thermal expansion coefficients. It follows that, due to the asymmetry and non-correspondence to the neutral axis, as well to as the integral connection between the structural element 2 and the guide elements 3, component deformation is generated, in particular a curvature thereof, and therefore geometrical changes in the axis (FIG. 4), which induce measurement errors.